1. Field of the Invention
The present invention relates to electronic samplers, and more particularly to high speed diode samplers for wideband radar receivers utilizing equivalent time sampling and other applications.
2. Description of Related Art
High range resolution systems such as wideband radar, time domain reflectometers, and recently developed radio location systems as described in U.S. Pat. No. 5,589,838 by McEwan often employ sampling type receivers. Typically, sampling receivers repetitively sample the incoming signal with sample timing that incrementally increases in delay up to a maximum and then begins over again at a minimum delay point. When the timing delay is slowly incremented relative to the RF pulse rate, the sampler output is a time-expanded replica of the RF waveform, appearing on a much slower time scale called equivalent time. Equivalent time systems are attractive from a standpoint of cost and performance, since wide bandwidth signals can be accurately processed at much lower bandwidths, usually in the audio frequency range.
Prior art samplers often use a 4-diode bridge structure where the bridge serves as a series gate between the RF input and the charge-holding capacitor. This structure and its deficiencies are discussed in U.S. Pat. No. 5,345,471 by McEwan. Generally, the 4-diode structure requires complementary gate pulses, requiring a broadband balun, and some of the signal is shunted away during sampling. Similar deficiencies exist in most 2-diode samplers.
McEwan, in U.S. Pat. Nos. 5,345,471 and 5,523,760 discloses integrating wideband samplers. These samplers require an impulse type gate function at the drive node of the sampling diodes. This is a general limitation since most impulse functions are derived from a step-like waveform produced by a fast switch. Preferably, the sampler can be driven directly by a step function.
Another limitation to virtually all prior art wideband samplers is the need to drive a relatively low impedance in the gating path, such as 50 .OMEGA.. It would be far better if the gate pulse generator could drive a very high impedance, or equivalently, a very small capacitance. A lightly loaded fast switch can switch rapidly even with substantial parasitic inductance in its path. Fast gate pulse transitions lead directly to wider sampling bandwidths.